Ethos is persuasion through the speaker's character — their credibility, judgement, and apparent good faith. Aristotle considered it the most potent of the three appeals, because an audience that trusts the speaker will accept a great deal on less evidence than they would otherwise require.
It is constructed, not merely possessed.
Antony, before dismantling Brutus: “I am no orator, as Brutus is; but, as you know all, a plain blunt man.” He is an orator, and the speech he is delivering proves it. The disclaimer is the construction.
Ethos divides into the credibility a speaker brings with them and the credibility the text itself builds. Only the second is within a writer's control.
Becomes Appeal to Authority when standing is offered in place of an argument.
Ethos is unavoidable. Every piece of writing projects a character — careful or careless, fair or partisan — and readers use that impression to decide how much scrutiny the content requires. Writing that denies it is doing rhetoric is simply doing it inattentively.
The legitimate version is earned in the text: accurate citation, fair statement of opposing views, claims scaled to the evidence behind them. These build credibility precisely because they would be costly to fake consistently.
The illegitimate version offers standing in place of reasoning. The difference is not whether authority is invoked but what it is asked to do. A credentialled source summarising the state of a field is evidence. The same credential offered as a reason to stop asking is not, and the fact that the person is genuinely eminent does not change which of the two is happening.
